1. Field of the Invention
The present invention relates to a regenerating method of etching solution (phosphoric acid aqueous solution) used for etching a silicon nitride film, an etching method and an etching system, in particular, relating to a technology to eliminate always effectively silicon compounds (a reactive product of silicon nitride with phosphoric acid) contained in an etching solution by etching treatment.
2. Description of the Related Art
In the etching treatment for mass-production lines including those for various substrates, etching is continuously carried out while etching solution is being filter-circulated to remove foreign materials such as debris to keep the etching solution in the tank clean. Such operation is similarly applied to the etching treatment of a silicon nitride film by heated phosphoric acid aqueous solution (etching solution), wherein etching solution is subject to filter-circulation to eliminate silicon compounds crystallized in the etching solution, together with other foreign materials by filter-purification for continuous etching performance. (Refer to JP 3-20895 B (Patent Document 1))
Said method, however, involves a problem of decreased etching rates resulting from raised concentrations of silicon compounds in the etching solution as a consequence of repeated treatments with the same etching solution. In order to solve this problem, a removal method of silicon compounds in the etching solution is proposed, in which a filter with cooling function is used to cool down the filter to raise removal efficiency. (Refer to JP 9-219388 A (Patent Document 2))
As an alternative way, a method is proposed wherein purified water is supplied to a filter which filtrates the etching solution to remove deposited silicon compounds by means of dissolving. (Refer to JP 6-310487 A (Patent Document 3))
As a further alternatively proposed way, temperature distribution of phosphoric acid as etching solution is made non-uniform to raise removing efficiency by adding water just before the etching solution is filtrated. This method proposes an efficient use of filters for the etching solution by installing these filters in parallel, wherein one filter is used for rinsing purpose at the time of replacing phosphoric acid aqueous solution. (Refer to JP 2005-260179 A (Patent Document 4))
However, the methods or the systems for regenerating etching solution described in JP 3-20895 B (Patent Document 1), JP 9-219388 A (Patent Document 2) and JP 6-310487 A (Patent Document 3) need to remove silicon compounds deposited within the filter by way of water washing, etc., since silicon compounds which gradually deposit within the filter during operation cause filters to clog. For such removal, operation of the system must be stopped periodically, leading to extremely inefficient performance.
On the other hand, according to the method described in JP 2005-260179 A (Patent Document 4), the filters are arranged in parallel and if silicon compounds deposit and clog a filter in service, the clogged filter is switched by a valve to washing operation with water, and at the same time, the other filter is supplied with etching solution, without interrupting operation of the system.
However, it was found by the inventor's experiments that the removal amount of silicon compounds by the filter increased with the increase of silicon compounds deposited in the filter. Namely, it was found that the removal amount of silicon compounds by a new filter element with no deposited silicon compounds or a filter element with no deposited silicon compounds right after rinsing for removal of deposited silicon compounds was extremely small, and the removal amount of silicon compounds increases operating time of the filter up to even more than 10 times. However, if the filter continues to be operated beyond a certain time range, the filter clogged and became unserviceable.
Because of these reasons, in the regeneration process of the etching solution described in Patent Documents 1-4, the filters with a low removal rate have to be always used for a certain period of time from the operation start of a filter and from the resuming operation after washing. This need is same either with the case in which the system is shutdown for filter cleaning and or with the case in which the filter elements are arranged in parallel and operation is continued without interruption by switching.